Dry Floodproofing

Homeowners considering alternatives for protecting their home from flood damage might consider wet floodproofing, dry floodproofing, and protective barriers. These methods have advantages and disadvantages to consider. They may fail if the wind and water forces and debris impacts are too great. “Floodproofing” does not mean that damage is not going to happen. It means the damage may be reduced and that the structure is made more flood or water resistant.

In addition, there are other alternatives including moving to a home outside the risk area, moving the structure itself to another location or elevating the structure. Before making a decision, contact local, area building officials, and flood plain managers about regulations. This module covers dry floodproofing.

Dry floodproofing means that techniques are applied to keep floodwaters from entering the structure. This involves sealing the structure to keep floodwaters out and to keep the structure dry below the flood protection elevation. However, keeping water out is difficult. FEMA guidelines suggest it may not be effective. Other professionals indicate that dry floodproofing is not acceptable for residential housing (Wendt & Aglan, Hurricane Season, 2006).

Research conducted at Tuskegee University shows that when the water level outside rose to 1 foot higher than the floor level, the dry flood proofed test structure started to leak. They managed to keep the water out for about ½ hour longer than if it had not been dry floodproofed.

Outdoor water level about 1 foot above inside floor level. Research at Tuskegee University. Photos courtesy of H.A. Aglan, PE, Ph.D., Tuskegee University.

Water entered the dry floodproofing test structure when the outdoor level rose about 1 foot higher than the inside floor level. Research at Tuskegee University. Photos courtesy of H.A. Aglan, PE, Ph.D., Tuskegee University.

Floodwater creates pressure on the exterior walls and floors that may cause the structure or walls to collapse and the floors to buckle. Water pressure and buoyancy underneath the structure may buckle the floor. When floodwaters are higher than about 2 to 3 feet, non-reinforced masonry and masonry veneer walls are much more likely to crack or collapse. Water may also enter through the floor.

Generally, dry floodproofing is used when the expected flood depths are low such as a few inches of water. The ability of the house structure to withstand the exterior flood pressure depends on the wall construction and the depth and movement of the water among other factors. Dry floodproofing is not recommended for houses with frame walls at the lower levels, or for houses with basements and crawlspaces.

Dry flood proofing is dependent in part on:

• Flood depth and length of time structure is flooded

• Potential for heavy floodwater movement or currents and wave action

• Potential for debris in the floodwaters or with high winds

• Wall construction and foundation

Things to think about when considering dry floodproofing:

• A structural engineer should be consulted before attempting this method.

• Dry floodproofing involves several steps such as anchoring the building, strengthening walls, raising the utility services and equipment above flood level, installing sump pumps, foundation drainage, etc.

Raise equipment above potential flood level. Photo: University of Nebraska-Lincoln, Shirley Niemeyer

Raise filter above potential flood level. Photo: University of Nebraska-Lincoln, Shirley Niemeyer

• Watertight shields are used to block off windows, doors, and other openings before the flood. This requires someone to do this before flooding occurs.

• Install check or backwater valves in drains to prevent floodwater from backing up into the drains of the home. A backwater gate or valve is installed at the end of a drain or outlet pipe to prevent the backflow of water or wastewater into the basement area. Used for systems in which sewer and street runoff drains share the same drain pipe system within a city and create backups into homes.

Backwater valve in drains helps to prevent backup from sewer and water overflow. Courtesy of Plastic Trends, Inc., Royal Pipe Systems

• The house must not be occupied during a flood.

• Dry floodproofing does not reduce the potential damage from wave action and heavy floodwater and currents.

• Maintenance is required.

• Flood insurance costs are not reduced.

• If the protection fails or the flood protection elevation is exceeded, the house will receive similar damage as if it was not protected.

• If the water pressure exceeds what the walls can withstand, the walls may buckle or collapse. Floors may buckle. This damage could be greater than if the water had been allowed to enter.

Summary

Two alternatives for protecting a home from flood damage include wet floodproofing and dry floodproofing. Both of these methods have advantages and disadvantages to consider. If you have National Flood Insurance, regulations must be considered. It is important to contact local zoning and code offices, local or area Emergency Management Agency as well as housing professionals and construction engineers when considering these alternatives. Specific codes and requirements are in place by the National Flood Insurance Program and must be met. Some examples of additional information sources are listed here.

For more information about dry and wet floodproofing:

American Red Cross Organization. Repairing Your Flooded Home, Chapter 8.

Federal Emergency Management Agency. Homeowner's Guide to Retrofitting: Six Ways to Protect Your House from Flooding, Chapters 3 and 7.

Wendt, R. & Aglan, H. (2006.) Rebuilding Your Flood-Damaged Home. Home Energy. pp. 20-26.